The present invention relates to an x-ray protection circuit for a video processor to prevent excessive voltages on the EHT (extra high tension) of the rube of a television set.
In particular, the present invention relates to an improved protection circuit which utilises a xe2x80x9cpower-on-resetxe2x80x9d circuit, to ensure that the x-ray output would not be held in the wrong state unable to automatically correct itself. Such a circuit enables automatic correction without the requirement of having to switch the power off and then on again, on the television set.
X-ray protection circuits are used in video processors to prevent excessive voltage on the EHT (extra high tension) of the rube of a television set. They detect the line flyback amplitude, and disable the horizontal output pulse when the amplitude of the flyback exceeds a certain limit.
A prior art protection circuit is shown in FIG. 1.
The circuit of FIG. 1 comprises a band gap comparator 1, which has a reference voltage input typically 1.3V, and, an x-ray input. When the x-ray input exceeds the (typically 1.3V) threshold, this triggers an analog d-latch (thyristor structure) 2 such that its output (the x-ray output) will be high. The analog d-latch will memorise the x-ray detection, and will do so until the supply voltage Vcc is reduced to below a predetermined value, typically 3V.
A problem with such prior art circuits is that the analog d-latch (thyristor structure) is easily triggered by parasitic spikes generated by the ESD (electrostatic discharge). This results in such protection circuit being held in the wrong state, and it is not able to automatically correct itself. To correct this occurrence the television is required to be switched off and then on again, such that the analog d-latch is reset.
In addition to the aforementioned prior art, the following documents describe x-ray protection circuits.
GB 2 274 377 describes a circuit which aims to filter the x-ray triggering obtained by the normal comparator input. This is done by counting (54) a certain number of trigger signals within a certain time interval before shutting down the horizontal deflection (through Qb). The RESET (56) of the counting operation is done if a certain number of pulses have not been counted, within a certain time interval. It requires a micro controller (4) for the operation, and the micro controller can be affected by ESD discharges. It also does mention ESD problems which can appear at any mode, such as Supply ground or substrate. ESD can lead to multiple parasites, which would still trigger the x-ray protection in this case.
U.S. Pat No. 4,074,323 describes a circuit including a thyristor (10) triggered by a voltage source derived from the winding (5c) of the flyback transformer (5). The filtering of the wrong signals is done with a RC circuit (20) and (21). The reset (23) is a (manual) switch activated after repair (page 4, lines 6-9). The memory system (10) is here a thyristor (external component). It also does nor mention ESD problems which can appear at any node, such as Supply ground or substrate.
The present invention seeks to provide an x-ray protection circuit for a video processor, which overcome the disadvantages of the prior art.
The present invention also seeks to provide an x-ray protection circuit for a video processor which ensures that the x-ray output is provided in a safe-state.
The present invention also seeks to provide a device which resets the protection circuit when an input exceeds a reference voltage or when a parasitic spike is detected on a supply voltage (Vcc), on ground or on a substrate.
In one broad form, the present invention provides an x-ray protection circuit to disable a horizontal output pulse of a video processor, characterised in that said protection circuit includes a reset circuit (5) to initialise said protection circuit whilst power of said video processor is switched on, and when a voltage level on the x-ray input exceeds a reference voltage or when a parasitic spike is detected on a supply voltage (Vcc), on ground or on a substrate.
Also preferably, the x-ray protection circuit further includes:
a comparator (6), to provide a comparator output signal when an x-ray input voltage exceeds a reference voltage; and
a bistable device (4), to receive said reset signal and said comparator output signal, and provide a memorised x-ray output signal to disable the horizontal output pulse when a voltage level on the x-ray input exceeds a reference voltage or a parasitic spike is detected on the supply voltage (Vcc), on ground or on substrate.
Preferably, said comparator is a bandgap comparator.
Also preferably, said reference voltage is about 1.3V.
In preferred forms, said bistable device is an analog or digital memory circuit with two stable states.
Preferably, said bistable circuit is a flip-flop.
Perhaps more preferably, said bistable circuit is an RS flip-flop.
In a preferred embodiment, said bistable circuit (4) gives priority to the reset (R), when set (S) and reset (R) are actived simultaneously due to a parasitic spike in supply voltage (Vcc), on ground or on a substrate.
In a most preferred form, said reset circuit is connected to the reset (R) input, said comparator is connected to the set (S) input, and said x-ray output is connected to the output Q of said RS flip-flop.
Also preferably, output QB of said RS flip-flop is connected to the BUS, the BUS being connected to the reset (R) input to additionally recover the x-ray if it is wrongly triggered by the parasitic spike.
In another preferred form, the present invention provides an x-ray protection circuit for a video processor including:
a reset circuit (5), to detect the occurrence of a parasitic spike or a voltage level on the x-ray input of said video processor, on a supply voltage (Vcc), on ground or on a substrate, exceeding a reference voltage, and provide a reset signal in response thereto;
a comparator (6), to provide a comparator output signal when a parasitic spike is detected or when an x-ray input voltage exceeds a reference voltage; and,
a bistable circuit (4), to receive said reset signal and said comparator output signal and provide a memorised x-ray output signal to disable the horizontal output pulse when a parasitic spike has occurred or when a voltage level on the x-ray input, on a supply voltage (Vcc), on a ground or on a substrate, has exceeded a reference voltage.
Preferably, said bistable circuit gives priority to the reset (R), when set (S) and reset (R) are activated simultaneously.
Also preferably, said bistable circuit is a modified RS flip-flop.